MDEV-19127 Assertion `row_start_field' failed in vers_prepare_keys upon ALTER TABLE

Prevent conflicting clauses at parser level.

Clear HA_VERSIONED_TABLE flag for DROP SYSTEM VERSIONING (for the sake
of strictness).

mysql-test/suite/versioning/r/alter.result

@@ -629,5 +629,14 @@ alter table t1 drop column `row_start`, drop column `row_end`, drop system versi
 ERROR 42000: Can't DROP COLUMN `row_start`; check that it exists
 alter table t1 drop column `row_end`;
 ERROR 42000: Can't DROP COLUMN `row_end`; check that it exists
+#
+# MDEV-19127 Assertion `row_start_field' failed in vers_prepare_keys upon ALTER TABLE
+#
+set system_versioning_alter_history=keep;
+create or replace table t1 (f1 int) with system versioning;
+alter table t1 add f2 int with system versioning, drop system versioning;
+create or replace table t1 (f1 int) with system versioning;
+alter table t1 drop system versioning, add f2 int with system versioning;
+ERROR HY000: Table `t1` is not system-versioned
 drop database test;
 create database test;

mysql-test/suite/versioning/t/alter.test

@@ -528,5 +528,15 @@ alter table t1 drop column `row_start`, drop column `row_end`, drop system versi
 --error ER_CANT_DROP_FIELD_OR_KEY
 alter table t1 drop column `row_end`;
 
+--echo #
+--echo # MDEV-19127 Assertion `row_start_field' failed in vers_prepare_keys upon ALTER TABLE
+--echo #
+set system_versioning_alter_history=keep;
+create or replace table t1 (f1 int) with system versioning;
+alter table t1 add f2 int with system versioning, drop system versioning;
+create or replace table t1 (f1 int) with system versioning;
+--error ER_VERS_NOT_VERSIONED
+alter table t1 drop system versioning, add f2 int with system versioning;
+
 drop database test;
 create database test;

sql/sql_yacc.yy

@@ -7368,6 +7368,11 @@ serial_attribute:
           {
             Lex->last_field->versioning= $1;
             Lex->create_info.options|= HA_VERSIONED_TABLE;
+            if (Lex->alter_info.flags & ALTER_DROP_SYSTEM_VERSIONING)
+            {
+              my_yyabort_error((ER_VERS_NOT_VERSIONED, MYF(0),
+                       Lex->create_last_non_select_table->table_name.str));
+            }
           }
         ;
 
@@ -8498,6 +8503,7 @@ alter_list_item:
         | DROP SYSTEM VERSIONING_SYM
           {
             Lex->alter_info.flags|= ALTER_DROP_SYSTEM_VERSIONING;
+            Lex->create_info.options&= ~HA_VERSIONED_TABLE;
           }
         | DROP PERIOD_SYM FOR_SYSTEM_TIME_SYM
           {

sql/sql_yacc_ora.yy

@@ -7292,6 +7292,11 @@ serial_attribute:
           {
             Lex->last_field->versioning= $1;
             Lex->create_info.options|= HA_VERSIONED_TABLE;
+            if (Lex->alter_info.flags & ALTER_DROP_SYSTEM_VERSIONING)
+            {
+              my_yyabort_error((ER_VERS_NOT_VERSIONED, MYF(0),
+                       Lex->create_last_non_select_table->table_name.str));
+            }
           }
         ;
 
@@ -8435,6 +8440,7 @@ alter_list_item:
         | DROP SYSTEM VERSIONING_SYM
           {
             Lex->alter_info.flags|= ALTER_DROP_SYSTEM_VERSIONING;
+            Lex->create_info.options&= ~HA_VERSIONED_TABLE;
           }
         | DROP PERIOD_SYM FOR_SYSTEM_TIME_SYM
           {